Mullite (3Al.sub.2 O.sub.3 *2SiO.sub.2) is a material (typically provided in granular form) that is widely used in the manufacture of refractory products and components. In this respect, mullite is the only alumina-silica compound that remains stable at very high temperatures, neither melting nor showing any noticeable reaction until a temperature of about 3270.degree. F. (1800.degree. C.) is reached. As a result, mullite grain/grog is typically used as the basic component in refractory products for high temperature applications.
Basically, every alumina-silica complex will form some mullite when fired to temperatures exceeding 2192.degree. F. (1200.degree. C.), but the actual amount formed not only depends on the firing temperature and the ratio of alumina to silica, but is also influenced enormously by the form in which both components are introduced, and by the kind and quantity of impurities present. For example, one refractory body containing 69% alumina (Al.sub.2 O.sub.3) and 27% silica (SiO.sub.2) may produce 89% mullite when fired, while another containing 71% alumina and 23% silica produces less than 35% mullite when fired.
It is known to form mullite by calcining kaolin clay. When pure kaolin clay is fired, it typically produces about 55% mullite. The mullite is in the form of crystals that are bonded by two forms of silica, i.e., cristobalite (which is one of the polymorphic crystalline forms of silica) and amorphous silica. The cristobalite forms about 15% of the resulting material of the fired or calcined kaolin, and the amorphous silica forms about 30% of the resulting material.
It is generally known that during rapid heating or cooling in the temperature range of 200.degree. C. to 250.degree. C. cristobalite undergoes a high/low inversion that is attended by a large change in volume. This crystallographic change of cristobalite makes mullite grain having cristobalite therein highly undesirable for high-temperature refractory materials and products, in that such a volume change may result in cracking of the refractory product. It is therefore highly desirable to produce a mullite grain/grog by calcining kaolin such that the final mullite grain/grog is free of cristobalite.
It is generally known that certain naturally occurring English kaolin clay, when calcined to high temperatures (i.e., in the neighborhood of approximately 2790.degree. F.) for long periods of time (approximately 60 hours) yields a substantially cristobalite-free mullite grain, having about 1% cristobalite. The absence of cristobalite in calcined English kaolin clay, that is fired as described above, is attributed to the presence of impurities in the clay, which impurities react with silica at elevated temperatures. Specifically, it is believed that naturally-occurring impurities found in natural English kaolin clay that have an affinity for silica react with the cristolbalite present to dissolve same during the prolonged soak period. The prohibitive energy costs of firing such clay for such a prolonged period of time make use of English kaolin clay and the aforementioned process less than desirable in producing cristobalite-free mullite.
It is also known to produce cristobalite-free mullite from clay by adding a suitable amount of alumina thereto and heating the clay to very high temperatures (in the range of about 3200.degree. F.). In this process, the additional alumina reacts with available silica to form additional mullite. However, this process is also cost-intensive due to the alumina additive and the energy required for firing at such high temperatures.
The present invention overcomes these and other problems and provides a method of producing cristobalite-free mullite grain, in substantially less time with substantially less energy than known heretofore, and at the same time, produces a mullite grain that is bonded by an extremely viscous, highly siliceous amorphous matrix.